This invention relates to a process for treating phospho gypsum.
The manufacture of phosphoric acid, according to the wet process, consists in essence of reacting calcium phosphate rock with sulfuric acid. The calcium oxide in the rock reacts with the sulfuric acid to produce calcium sulphate di-hydrate, also known as gypsum. This calcium sulphate crystallises out in the phosphoric acid medium in the reactor. Since this gypsum is a by-product of the phosphoric acid production process it is commonly known as phospho gypsum.
The phospho gypsum is filtered and rinsed with water to recover as much phosphoric acid as is economically justified. However, the phospho gypsum typically contains water soluble P2O5 bound in the crystal lattice which is not recovered as phosphoric acid during the rinsing stage. As a result, phosphoric acid production plants lose substantial amounts of phosphoric acid, expressed as P2O5 bound in the gypsum crystal lattice.
In the manufacture of Portland cement, gypsum is interground with clinker in order to regulate the setting and hardening process of the cement after addition of water. When phospho gypsum is used for this purpose it is found that the setting times are extended to such a degree as to interfere with normal operations on a construction site. It has been found that these extended setting times are caused by the water soluble P2O5 in the gypsum.
Whilst it is possible to convert the water soluble P2O5 adhering to the outside of the gypsum particles to insoluble phosphates, by washing with a calcium hydroxide solution at ambient or elevated temperatures, the P2O5 in the crystal lattice is not accessible to the hydroxide under these conditions. Hence phospho gypsum treated in this manner gives rise to variable setting times when used in cement production.
Several processes exist to free the P2O5 in the crystals. Some consist in essence of drying the gypsum and heating same to high temperatures in order to break up the crystal structure thus releasing the P2O5. Limestone, having been added, dissociates at these high temperatures forming calcium oxide which then reacts with the P2O5 to form water insoluble calcium phosphates. The product, which consists of calcium sulphate anhydrite and calcium phosphate, is then cooled and sprayed with water to convert the anhydrite back to calcium sulphate hydrates. It should be noted that all P2O5 in the crystals is converted to insoluble phosphates and is thus lost to the phosphoric acid production process. It is clear that this process, and some variations of same, is energy intensive because of the drying and subsequent dehydration of the gypsum and the heat required to dissociate the limestone. Capital outlay is also high. Hence this process is economically not attractive.
According to one aspect of the invention there is provided a process for recovering phosphoric acid from phospho gypsum produced as a by-product in a conventional phosphoric acid production plant and having water soluble P2O5 bound in the crystal lattice thereof, the process comprising:
a) forming a suspension of phospho gypsum in an aqueous medium;
b) subjecting the suspension to ultrasonic waves under conditions suitable to shatter the phospho gypsum crystal lattice releasing the bound water soluble P2O5 into the aqueous medium; and
c) recovering the phosphoric acid so released.
The depleted phospho gypsum is preferably treated in a further step to reduce any remaining water soluble P2O5 to a level which allows the gypsum to be added to a cementitious material for use as a setting regulator.
The level of water soluble P2O5 in the gypsum is preferably reduced to below 0.06% by weight, in particular to about 0.01% by weight.
The suspension is preferably subjected to ultrasonic waves of less than 2 MHz, typically 50 to 500 Hz or 16 kHz to 2 MHz.
During the subsequent treatment of the phospho gypsum, typically sonication of the depleted phospho gypsum in an aqueous medium, a neutralising agent may be added to the aqueous medium to convert the water soluble P2O5 into an insoluble phosphate.
According to a further aspect of the invention, there is provided a cement comprising phospho gypsum treated in accordance with the invention.
The crux of the invention is a process for dislodging P2O5 bound in the crystal structure or lattice of phospho gypsum, recovering the dislodged P2O5, and the further treatment of the depleted gypsum to render it suitable for use in either the cement industry or the gypsum products industry or both.
It has been reported in the literature that ultrasonic waves of suitable wavelength and intensity cause cavitation in a liquid medium. The small bubbles collapse giving rise to localised xe2x80x9chotspotsxe2x80x9d having very high temperatures and pressures, in addition to shockwaves. It has now been found that this phenomenon can be used for the shattering of a phospho gypsum crystal lattice thus releasing P2O5 bound therein.
In general the process consists of forming a suspension or slurry of phospho gypsum derived as a by-product in a conventional phosphoric acid production plant in an aqueaus medium, typically water, and subjecting the suspension to ultrasonic waves of a suitable wavelength. This process leads to the disruption or shattering of the phospho gypsum crystal lattice thus releasing the water soluble P2O5 bound in the crystal lattice to form phosphoric acid. Typically a wavelength of less than 2 MHz will be used to break up the crystal lattice. Examples of suitable wavelength ranges are 50-500 Hz and 16 kHz-2 MHz, based on economic and end use requirements.
Subsequent filtration and drying at low temperatures produces a gypsum product eminently suitable for the cement industry and for the general gypsum products industry whilst providing for the economic recovery of phosphoric acid.
Whilst laboratory tests have shown that a wide range of frequencies can be used, a sonicator suitable for industrial purposes has been found to be the patented Nearfield Acoustic Processor (NAP) manufactured by the Lewis Corporation of the USA. This machine has two opposing diaphragm plates vibrating at two different frequencies, namely 16 kHz and 20 kHz. These plates form the two active walls of the reaction chamber through which the slurry is pumped. The intensity of the vibrations can be adjusted as required, whilst the gap between the plates can be varied between 0.12 and 25.4 mm.